We report the development of a defect-engineered thermoelectric material using Si_1−xGe_x alloys grown on a c-plane sapphire substrate via electron beam (E-beam) irradiation. This paper outlines the idea of growing the Si_1−xGe_x film at relatively high temperatures to obtain good crystalline properties, then controlling the amount of twins or dislocations through ex situ electron-beam irradiation. The current work suggests that structure reconstruction by bond rearrangement through E-beam irradiation may be used for tailoring thermoelectric properties.